check method timing threshold when observing method performance

lib/gitlab/metrics/method_call.rb

@@ -59,7 +59,7 @@ module Gitlab
         @cpu_time += cpu_time
         @call_count += 1
 
-        if prometheus_enabled?
+        if prometheus_enabled? && above_threshold?
           self.class.call_real_duration_histogram.observe(@transaction.labels.merge(labels), real_time / 1000.0)
           self.class.call_cpu_duration_histogram.observe(@transaction.labels.merge(labels), cpu_time / 1000.0)
         end
@@ -87,7 +87,7 @@ module Gitlab
       end
 
       def prometheus_enabled?
-        @prometheus_enabled ||= current_application_settings(:prometheus_metrics_method_instrumentation_enabled)
+        @prometheus_enabled ||= Gitlab::CurrentSettings.current_application_settings[:prometheus_metrics_method_instrumentation_enabled]
       end
     end
   end

spec/lib/gitlab/metrics/method_call_spec.rb

@@ -13,6 +13,17 @@ describe Gitlab::Metrics::MethodCall do
       expect(method_call.call_count).to eq(1)
     end
 
+    context 'when measurement is above threshold' do
+      before do
+        allow(method_call).to receive(:above_threshold?).and_return(true)
+      end
+
+      context 'prometheus instrumentation is enabled' do
+        before do
+          allow(Gitlab::CurrentSettings).to receive(:current_application_settings)
+                                              .and_return(prometheus_metrics_method_instrumentation_enabled: true)
+        end
+
         it 'observes the performance of the supplied block' do
           expect(described_class.call_real_duration_histogram)
             .to receive(:observe)
@@ -26,6 +37,44 @@ describe Gitlab::Metrics::MethodCall do
         end
       end
 
+      context 'prometheus instrumentation is disabled' do
+        before do
+          allow(Gitlab::CurrentSettings).to receive(:current_application_settings)
+                                              .and_return(prometheus_metrics_method_instrumentation_enabled: false)
+        end
+
+        it 'does not observe the performance' do
+          expect(described_class.call_real_duration_histogram)
+            .not_to receive(:observe)
+
+          expect(described_class.call_cpu_duration_histogram)
+            .not_to receive(:observe)
+
+          method_call.measure { 'foo' }
+        end
+      end
+    end
+
+    context 'when measurement is below threshold' do
+      before do
+        allow(method_call).to receive(:above_threshold?).and_return(false)
+
+        allow(Gitlab::CurrentSettings).to receive(:current_application_settings)
+                                            .and_return(prometheus_metrics_method_instrumentation_enabled: true)
+      end
+
+      it 'does not observe the performance' do
+        expect(described_class.call_real_duration_histogram)
+          .not_to receive(:observe)
+
+        expect(described_class.call_cpu_duration_histogram)
+          .not_to receive(:observe)
+
+        method_call.measure { 'foo' }
+      end
+    end
+  end
+
   describe '#to_metric' do
     it 'returns a Metric instance' do
       method_call.measure { 'foo' }